A definition from wikipedia:
"In computing, a game engine is the core software component of a video game. It typically handles rendering and other necessary technology, but might also handle additional tasks such as game AI, collision detection between game objects, etc. The most common element that a game engine provides is graphics rendering facilities (2D or 3D)."
Quote: "Could you write, say, a 2D engine in DBC?"
Answer:
Every time you code a program with 2D components you create a 2D Engine that handles the images in the way you decided in the code.
Quote: "If they are functions are they rendering, physics, or file accessing functions?"
Answer:
All of them and even more(sounds,memblocks,vectors etc..).
"An eye for an eye will only make the whole world blind."